The present invention relates to an optical element and an optical pick-up device which are used when recording or reproducing of information for an optical recording medium such as an optical disk is performed, and to a manufacturing method of the optical element.
In the past, there has been proposed an optical information recording media such as an exclusive optical disk for reproducing, a phase changing type optical disk, a photo-electro-magnetic disk or an optical card, as the recording media for storing the data of video information, audio information or a program for a computer. Further in the recent years, there are strong demands for improving the recording density and for giving a large capacity to these optical information recording media.
For the means to improve the recording density of the optical information recording media, in an optical pick-up device which performs writing and reading of an information signal, it is possible to make a spot diameter of light converged by the light-converging optical element to be smaller, by making a numerical aperture (NA) of a light-converging optical element to be larger, or by shortening the wavelength of a light source.
In an optical recording using near-field light proposed in the recent years, by forming a very small spot less than the wavelength of light by using a probe or SIL (Solid Immersion Lens) having a very small aperture less than the wavelength of light, and by performing the recording or reproducing by making the clearance between an optical head and a recording medium to be less than the wavelength of light source, it is possible to write and read a small mark as a signal less than 200 nm beyond the light diffraction limit.
Here, the near-field light and near-field light recording using the same are explained, referring to FIG. 1. When considering the light-converging optical system wherein light emits from a medium of an optical element (refractive index: n) to an air layer (refractive index: 1, wavelength of light source: λ, thickness of air: d, d≦λ) with a prescribed angle θ to the optical axis and wherein light converges onto the information recording surface, it is generally known that when angle θ goes beyond the total reflection critical angle θ1, light reflects totally and does not enter the air layer. In this case, since refractive index n is expressed by n=1 in the air layer, numerical aperture NA of an image side of the light-converging optical system is expressed by NA=n sin θ<1.0, and a spot diameter is λ/NA, and accordingly, it has been considered that it is impossible to stop down the spot diameter less than the range of the wavelength.
However, when looking this phenomenon from the view point of an electro-magnetic wave theory, totally-reflected light leaks once to an air layer area, which is called near-field light or evanescent light. The near-field light has the nature that the wavelength is shortened to λ/n, though the near-field light exists in the air layer. By establishing an air layer clearance between the optical element and the information recording surface to be less than wavelength of a used light source, it is possible to use the near-field light for the light converging, and the recording or reproduction of information can be performed. In this case, since n is greater than one (n>1) in the medium of the optical element, the numerical aperture NA of the image side of the light-converging optical system is written as NA=n sin θ>1.0. That is, it is possible to stop down the spot diameter to be less than the wavelength, which can perform the high density recording. Further, light intensity of a central spot of the near-field light attenuates exponentially for the distance of leakage as shown in FIG. 7. Due to this, in order to perform the correct reading and writing of information, it is necessary to make the distance between a final surface of the optical element and the recording medium to be about the wavelength of the light source, which is well known.
Incidentally, for the near-field light recording technology, there is proposed a SIM (Solid Immersion Mirror) of catadioptric system, including a flat shaped reflecting surface having a light transmitting section on the center, a reflecting surface representing a rotated ellipse surface which converges the light onto the above-mentioned light transmitting section, and a refracting interface representing a concave shape which transmits the light, positioned on the center of the above-mentioned reflecting surface. (TOKUKAI 2001-43550)
However, when there is used the so-called glass molding method, wherein a heat softened glass material is pressed by an upper metal mold and an lower metal mold to be the optical surface shape, is used for the mass-production of this kind of the optical element, the following problems will occur. As shown in FIG. 2, pre-form 1 (material of the optical element), representing a first processed goods being nearly a spherical shape when formed, is set on a position slipped from a center of lower metal mold 2A by projected section 2B which forms a concave refracting interface of the optical surface of the metal mold. Concerning optical element 1′ formed by the above-mentioned condition, an outermost diameter is off-centered to the optical axis, in a forced-out type molding method by which a forced-out flange becomes a free style, as shown in FIG. 3. This section is used for the positioning of a pick-up device to a lens barrel, which makes the optical surface to shift to the optical axis, and affects adversely to the optical characteristics. Further, in the above-mentioned forced-out type molding method or a filling type molding method having a molding surface on the flanged section, the distribution of pressing pressure at the formation is so uneven to the optical axis that shrinkages are generated, because contraction is concentrated to the portion where the pressure of the optical surface is lower, when the formed optical element is cooled to be solidified, which causes the generation of coma and astigmatism, and affects adversely to the optical characteristics. As for a countermeasure, it is conceivable that the upper metal mold and the lower metal mold of the optical surface are reversed to the case of FIG. 2, and taken is an arrangement in which a flat-shaped second reflective surface is formed by the lower metal mold, however, setting position of the pre-form is not fixed due to the flat surface, and the pre-form rolls easily, which causes the same problem.
As mentioned above, it is necessary that the distance between the optical element and the recording medium is determined to be the wavelength level or less for the near-field light recording, and for the technology that is possible to realize this, there has been proposed a flying head, used for a hard disk drive at present. The flying head is the one that stabilizes a floating amount at the clearance balanced between buoyancy, that is generated during the rotation of a disk by air influx between the disk and the optical element or a slider on which the optical element is loaded, and a load, that forces the disk surface by the optical element or an arm supporting the slider.
At present, it is general that a protective layer is provided on the surface of removable disks such as CD (compact disk) and DVD (digital versatile disk), for the purpose of protecting their recording layers from a dust and scratch. Here, when the protective layer for the recording medium is used for the optical system wherein NA is greater than one, like the near-field light recording, the generated amount of spherical aberration caused by a thickness error of the protective layer is very large. Further, focal depth is very small due to high NA, thereby the light spot diameter is deteriorated by the small change of a focal position, and due to these, the recording/reproducing for information cannot be performed correctly. Accordingly, it is necessary to perform focusing by deviating the converged optical system against unevenness of the thickness of the light-converging optical element or the thickness of the protective layer provided on the recording medium. However, concerning the near-field light recording, since the distance between the optical element for converging the light to the recording layer and the medium is established to be the same as or less than the wavelength level and is fixed, there has been a problem that the stroke for the focusing cannot be obtained in the light-converging optical element.